Heater wire coating process



April 10, 1962 R. W. ETTER HEATER WIRE COATING PROCESS Filed April 29, 1958 INVENTOR. Rosier I44 Err-m coating of the kind normally applied to a filamentary heater wire for use in an indirectly heated thermionic cathode of an electron discharge device.

crack with the result that electrical short circuits develop broken away having an electrically insulating heat conincludes a metal tube 12 of nickel or the like having a The heating filament comprises a high resistance electrically conducting wire 15, such as tungsten, shaped into United States Patent This invention relates to indirectly heated cathodes and, particularly, to an improved electrically insulating A conventional indirectly heated thermionic cathode comprises a metallic cathode sleeve having an electronemissive coating on its outer surface and a coiled heater 1.3

wire disposed within the metallic sleeve and adapted to heat the cathode to a temperature sufficiently high to cause the coating to emit electrons.

The heater wire in a conventional tube is coated with an electrically insulating material, such as aluminum oxide, to electrically insulate the turns of the heater wire from the cathode and from other turns of the heater.

Heater wires are subject to the criticism that the electrically insulating coating will frequently chip and/orbetween the turns of the wire or between the heater Wire and the cathode. In addition, the heater wire often sags and, if the sagging is severe enough, the cathode will not be heated uniformly, or heated to the desired temperature, since the relative spacing between areas of the cathode and the heater will be varied.

Furthermoreone or" the factors determining the useful life of an electron tube is the length of time during which the heating filament is capable of maintaining the desired temperature of the cathode.

The present invention is concerned with an improved coating for a filamentary heater wire, particularly for use in an indirectly heated type of cathode, characterized in its improved efliciency, decreased sagging and longer life.

In brief, according to the invention, a filamentary heat er Wire for use in an indirectly heated cathode is provided with an electrically insulating, heat conducting coating including aluminum oxide or boron oxide and a combination of at least two of the oxides of the transition elements of the periodic chart. The transition elements of the periodic chart, in accordance with this invention, consist of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, molybdenum, rhodium, tungstem, and tantalum. The electrically insulating coating is applied by cataphoresis from a solution including aluminum oxide or boron oxide and the other selected materials.

In the single sheet of drawings, the single figure is a sectional view of a cathode and heater filament, partially ducting coating thereon in accordance with this invention. Referring in detail to the drawings, the single figure is a sectional view of an indirectly heated cathode 10 which 3,029,360 Patented Apr. 10, 1962 Percent Aluminum oxide 4-3.6 Aluminum nitrate 0.9 Magnesium nitrate 0.9 Demineralized water 27.8 Ethyl alcohol 24.0 Chromic oxide 1.4 Titanium dioxide 1.4

The chromic oxide and titanium dioxide both may be provided in an amount within the approximate range of about 0.7 weight percent to about 1.4 weight percent. Other materials which may be provided, examples of which will subsequently be given, may be selected from the oxides of the transition elements of the periodic chart, as defined in this invention, and any combinations of two or more of these materials may be used.

The other components may be varied also; namely, aluminum nitrate and magnesium nitrate. Other alcohols, such methanol, propanol or ethylene glycol may also be employed.

At percentages below about 0.7 weight percent of the oxides of the transition elements, it has been found that the strength of the coating decreases. Also, at percentages greater than about 3.0 weight percent of these oxides, it

has been found that electrical leakage through the coating I wire 15, the heater wire to be coated is inserted into a the desired form and coated with a layer of electrically insulating thermal conducting material 16. According terials selected from the oxides of the transition elements of the periodic chart. The term transition elements, as

coating bath which is held in a suitable container.

A stainless steel electrode is connected to the positive terminal of a variable source (not shown) of directelectric" current with the heater wire 15 connected to the negative terminal. During the coating process, the coating bath may be stirred mechanically or by hand to maintain uniformity of the solution throughout the bath and to prevent settling out of the oxides. The stirring operation is carried out carefully to prevent excessive turbulence in the coating bath.

With an applied voltage of about to volts. a suitable coating of the oxides having a thickness of the order of 6 mils may be provided in about 7 seconds. After the heater has been coated, it is washed in acetone to remove excess coating and then dried in clean air. Next, the coated heater is placed in a molybdenum boat, which has previously been coated with aluminum oxide to, prevent contamination of the heater coating 16, and fired at a temperature within the approximate range of about 1375 C. to about 1825 C. The specific example given above was fired at about 1650 C. for about 3 to 5 minutes in a hydrogen atmosphere. The firing operation sinters the coating to the filament or heater wireQ s,029,aeo

3 Other examples of successfully constructed and operated coatings are as follows: i

Example 2.-Aluminurn oxide 47.5%, methanol 49.5%, chromic oxide 1.3%, titanium dioxide 1.3%,

magnesium nitrate 0.4%.

47.6% methanol 49.4%, zirconium dioxide 1.9%, manga ese dioxide 0.7%, magnesium nitrate 0.4%.

Example 8.Alurninurn oxide 47.7%, methanol 49.5%,cobalt oxalate 1.2%, titanium dioxide 1.2%, magnesium nitrate 0.4%.

Example 9.-Aluminum oxide 47.2%, methanol 49.3%, zirconium dioxide 1.9%, cobalt oxalate 1.2%,

magnesium nitrate 0.4%. 7

Example ]0.Aluminum oxide 46.5%, methanol 48.5%, water 2.0%, cohaltic oxide 1.4%, titanium dioxide 1.2%, magnesium nitrate 0.4%.

Example 11 .-4Aluminum oxide 46.4%, methanol 48.0%, Water 2.0%, zirconium dioxide 1.8%, cobaltic oxide 1.4%, magnesium nitrate 0.4%.

Example 12.,.Aluminum oxide 46.9%, methanol 48.5%, water 2.0%, titanium dioxide 1.4%, nickelic oxide 0.8%, magnesium nitrate 0.4%.

Example 13..-Aluminum oxide 46.5%, methanol 48.5%, water 2.0%, zirconium dioxide 1.8%, nickelic oxide 0.8% magnesium nitrate 0.4%.

Example 14,-Boric acid 48.2%, methanol 49.5%,

manganese dioxide 0.7%, magnesium nitrate 0.4%, ti-

tanium dioxide 1.2%. I

Example Her-Aluminum oxide 45.2%, methanol 47.5%, water 3.9%, zirconium dioxide 2.0%, nickelic oxide 1.2%, magnesium nitrate 0.2%.

Example ]6.Aluminum oxide 45.2%, methanol 47.5%, water 3.9%, tungstic acid 2.0%, titanium dioxide 1.2%, magnesium nitrate 0.2%.

Example 17.-Aluminum oxide 448%, methanol 47.2%, water 3.9%, tungstic acid 2.0%, zirconium dioxide 1.9%, magnesium nitrate 0.2%.

Example 18.--Aluminum oxide 45.2%, methanol 47.5%, water 3.9%, tungstic oxide 2.0%, titanium dioxide 1.2%, magnesium nitrate 0.2%.

Example 19.A luminurn oxide 45.0%, methanol 7 47.3%, water 3.9%, zirconium dioxide 2.0%, tungstic titanium dioxide 1.3%, magnesium nitrate 0.4%.

A heater wire coated as described above is provided with a strong chip-proof adherent coating possessing high resistance to crushing. Thus, the heaters may be handled without fear of breaking the coating during assembly and service of an electron tube. In fact, when the heater wire is in the form of a coil, it may be extended 4 somewhat or uncoiled appreciably without breaking away the sintered insulating coating.

An important additional advantage is that heaters coated according to this invention are practically completely resistant to sagging during operation of the tube. An auxiliary advantage of this invention is that a cataphoretic coating including chromic oxide and titanium dioxide heats the cathode more efficiently at lower heater temperature because of the dark-color, grey-black, and consequent higher emissivity of the coating. Also, the color difference produced by using the diilerent examples of the electrically insulating coating affords .a

means of cataloging the heaters as to size, resistance,

power ratings or the like.

What is claimed is:

1. A subassembly for an electron discharge device comprising an electron emissive cathode, a heater for said cathode comprising a conductive means having an electrically insulating coating thereon, said insulating coating including a material selected from the group consisting of aluminum oxide and boron oxide, said insulating coating further including oxides of at least two different materials selected from the group of materials consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, molybdenum, rhodium,.tungsten and tantalum, and each of said at least two oxides being present in an amount within the range of substantially 0.7 weight percent to substantially 3.0 weight percent.

2. A coating material for a heater wire comprising aluminum oxide and at least two other oxides selected from the transition elements of the periodic chart, and each of said two other oxides being present in an amount Within the range of substantially 0.7 weight percent to substantially 3.0 weight percent.

3. A heater wire having an electrically insulating thermal conducting coating thereon, said coating comprising,

in weight percent approximately 45.0 to 46.9 percent aluminum oxide, 47.2 to 48.5 percent alcohol, 0.2 to 0.4 0

percent magnesium nitrate, 2 to 4.5 percent water, and 0.8 to 2.0 percent oxides of two materials selected from x the group consisting of cObaIL t-itanium, zirconium, nickel,

tungsten, molybdenum, and iron.

4. A heater comprising a wire, a coating composition on said wire, said composition comprising approximately 47.2 weight percent aluminum oxide, approximately 48.8 weight percent methanol, approximately 2.3 weight percent molybdenum sesquioxide-nickelic oxide, approximately 1.3 weight percent titanium dioxide, and approximately 0.4 weight percent magnesium nitrate.

5. A heater comprising a wire, a-coat-ing composition on said wire, said composition comprising approximately 47.5 weight percent aluminum oxide, approximately 49.5 weight percent methanol, approximately 1.3 weight percent chromic oxide, approximately 1.3 weight percent titanium dioxide, and approximately 0.4 weight percent magnesium nitrate.

6. A heater comprising a wire, a coating composition on said wire, said composition comprising approximately 45.6 weight percent aluminum oxide, approximately 48,0 1

weight percent methanol, approximately 4.0 weight percent water, approximately 1.1 weight percent iron oxide, approximately 1.1 weight percent titanium dioxide, and approximately 0.2 weight percent magnesium nitrate.

7. A heater comprising a wire, a coating composition on said wire, said composition comprising approximately 45.0 weight percent aluminum oxide, approximately 47 .3 weight percent methanol, approximately 3.9 weight percent water, approximately 2.0 weight percent zirconium' dioxide, approximately 1.6 Weight percent tungstic oxide, and approximately 0.2 weight percent magnesium nitrate.

8. A heater comprising a wire, a coating composition on said wire, said composition comprising approximately 47.7 weight percent aluminum oxide, approximately 50.0 weight percent methanol, approximately 1.2 weight percent titanium dioxide, approximately 0.7 weight percent newmanganese dioxide, and approximately 0.4 weight percent magnesium nitrate. g

9. A heater comprising a wire, a coating composition on said wire, said composition comprising approximately 47.018 47.7 weight percent aluminum oxide, approximately 48.8 to 50.0 weight percent alcohol, approximately .4 weight percent magnesium nitrate and the balance being oxides of two materials selected from the group consisting of chromium, titanium, zirconium, vanadium, manganese, cobalt, molybdenum and nickel.

10. A heater comprising a wire, a coating composition on said wire, said composition comprising approximately 44.8 to 47.7 weight percent of a material selected from the group consisting of aluminum oxide and boron oxide, approximately 47.2 to 50.0. weight percent alcohol, approximately .2 to .4 weight percent magnesium nitrate,

approximately .7 to 2.3 weight percent each of the oxides of two different materials selected from the group consisting of chromium, titanium, zirconium, vanadium, manganese, cobalt, nickel, tungsten and molybdenum, and the balance water.

References Cited in the file of this patent UNITED STATES PATENTS Schrotter July 23, 1957 

1. A SUBASSEMBLY FOR AN ELECTRON DISCHARGE DEVICE COMPRISING AN ELECTRON EMISSIVE CATHODE, A HEATER FOR SAID CATHODE COMPRISING A CONDUCTIVE MEANS HAVING AN ELECTRICALLY INSULATING COATING THEREON, SAID INSULATING COATING INCLUDING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE AND BORON OXIDEM SAID INSULATING COATING FURTHER INCLUDING OXIDES OF AT LEAST TWO DIFFERENT MATERIALS SELECTED FROM THE GROUP OF MATERIALS CONSISTING OF TITANIUM, VANADIUM, CHROMIUM, MANGANESE, IRON, COBALT, NICKEL, ZIRCONIUM, MOLYBDENUM, RHODIUM, TUNGSTEN AND TANTALUM, 